Preparation of aromatic fluorides from corresponding aromatic primary amines by a diazotization-fluorination process is well known. In such processes, an aromatic primary amine is diazotized in a reaction mixture comprising (i) a reaction medium consisting essentially of hydrogen fluoride, (ii) the amine and (iii) a nitrosonium ion-containing or--generating diazotization agent under reaction conditions such that the resulting reaction mixture comprises an aromatic diazonium fluoride. The aromatic diazonium fluoride then is decomposed at elevated temperature in the resulting reaction mixture to nitrogen and the corresponding aromatic fluoride.
Generally, as is illustrated in U.S. Pat. No. 4,075,252, this decomposition step occurs by heating the reaction mixture to the decomposition temperature. The decomposition temperature is that temperature at which nitrogen evolution from the reaction mixture becomes substantially complete. Usually, as the reference discloses, nitrogen evolution will have been completed by the time the temperature reaches the reflux temperature of the reaction mixture. In those instances where higher temperatures are required, they can be obtained either by heating under pressure or evaporating solvent until the desired temperature is reached. U.S. Pat. No. 4,075,252 discloses the addition of ammonium salts to lower the reflux temperature.
U.S. Pat. No. 4,096,196, discloses the inclusion of tertiary amines in a diazotization reaction medium. This reference does not state how the addition of tertiary amine compounds aids the diazotization-fluorination reaction. Olah, et al., J. Org. Chem., Vol. 44, No. 22, 1979, discloses the use of pyridine in an HF solution (pyridinium poly(hydrogen fluoride) reagent) rather than pure HF as a fluorinating agent.
Yoneda, Synthetic Communications, 19:865-871 (1989) discloses the preparation of aromatic fluorides via diazotization in anhydrous hydrogen fluoride--organic base solutions.
European Patent Application 0330420 discloses a process for the manufacture of fluoroaromatics which consists of feeding an aromatic amino in the presence of HF to a reaction zone simultaneously with a diazotizing agent so as to effect diazotization of the aromatic amine, thermally decomposing the resulting diazonium salt substantially as it is formed, and removing the resulting fluoroaromatic compound from the reaction zone substantially as it is formed. The amine and the diazotization agent are fed to the reaction zone in such quantities and proportions that they are consumed substantially as fed so that no substantial concentration of either builds up in the reaction zone. A preferred embodiment discloses the addition of ammonium ions to the reaction mixture.
The use of urea as a nitrosonium scavenger in diazotization reactions is disclosed in U.S. patent application Ser. No. 151938, filed Feb. 3, 1988, incorporated herein by reference. Urea is not a nitrosonium scavenger in HF systems.
The heretofore known processes for preparing fluoroaromatic compounds via decomposition of corresponding aromatic diazonium fluorides have not been entirely satisfactory in that they typically are complex, inefficient, expensive, and/or prone to result in an unexceptably high level of tar and/or other by-products. Accordingly, there is a substantial need in the art for a diazonium fluoride decomposition process which would overcome these disadvantages. There is also a need for a process for decomposing aromatic diazonium fluorides which provides for the optimization of yield with the minimization of undesired by-products and avoids unnecessary safety hazards.